Phosphites represent a diverse class of organic phosphorus compounds that are useful as ligands for homogeneous catalysis and as components of plasticizers, flame retardants, UV stabilizers and antioxidants. Phosphites can be further classified as organomonophosphites and organopolyphosphites. Organopolyphosphites are particularly useful for certain homogeneous catalysis; for example, U.S. Pat. No. 4,769,498 generally relates to synthesis of organopolyphosphites and use thereof as ligands in hydroformylation processes.
Phosphoromonochloridites are intermediates for synthesizing organopolyphosphites; see, for example, U.S. Pat. Nos. 6,031,120; 5,663,369, and 4,769,498. A phosphoromonochloridite is typically synthesized in a condensation reaction by contacting phosphorus trichloride (PCl3) with one molar equivalent of a di-alcohol or two molar equivalents of a mono-alcohol under reaction conditions dependent upon the reactivity of the starting alcohol and the resulting phosphoromonochloridite. For each molecule of a phosphoromonochloridite produced, the condensation reaction produces two molecules of hydrogen chloride (HCl). In order for the condensation reaction to achieve high, for example, greater than 90 percent, conversion of the alcohol, HCl needs be removed from the reaction solution.
One approach for HCl removal from the condensation reaction is to neutralize HCl using a nitrogen base, in an amount stoichiometric to or in excess to the theoretical amount of HCl to be produced. See, for example, U.S. Pat. Nos. 5,235,113; 6,031,120, and 7,196,230, U.S. patent application publication 2007/0112219 A1, and Journal of Molecular Catalysis A: Chemical 164 (2000) 125-130. When a nitrogen base is used, however, the resulting nitrogen base-HCl salt must be removed from the reaction mixture by a filtration procedure, which generates chloride and nitrogen-containing wastes that, in turn, increase cost.
Another approach for HCl removal from the PCl3-alcohol condensation reaction involves heating a mixture of the alcohol and a large excess amount of the PCl3 at a temperature sufficiently high to reflux PCl3 (boiling point (bp): 74-78° C.), which drives off the HCl. A nitrogen base is not needed in this approach. For example, U.S. Pat. No. 4,769,498 discloses a procedure for producing 1,1′-biphenyl-2,2′-diyl phosphoromonochloridite by refluxing a mixture of 2,2′-biphenol with 3.7 molar equivalents (2.7 equivalents in excess) of PCl3. The product, 1,1′-biphenyl-2,2′-diyl phosphoromonochloridite, is disclosed to be isolated in 72 mole percent yield, based on moles of 2,2′-biphenol used, by distillation under reduced pressure. Another procedure, as referenced in Korostyler et al., Tetrahedron: Asymmetry, 14 (2003) 1905-1909, and Cramer et al., Organometallics, Vol. 25, No. 9 (2006) 2284-2291, synthesizes 4-chlorodinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine by heating a mixture of 1,1′-bi-2-naphthol and 11.5 molar equivalents of PCl3 at 75-80° C. One undesirable feature of the aforementioned approach is that it involves the need to remove and handle a large excess amount of PCl3, which reacts exothermically with moisture and typically involves additional safety considerations. It would be desirable to reduce the excess amount of PCl3 to be used in the process.
In view of the above, there is a need in the art for a more efficient process for producing phosphoromonochloridites.